Fix breaking JS on sample size calculator

The sample size calculator JS was breaking if query params were specified but one was missing.  This PR addresses extending the default object to account for missing query params.  

To test go to and make sure there are no JS errors:

http://optimizely-marketing-site-staging.s3-website-us-east-1.amazonaws.com/dfoxpowell/fix-sample-calculator-params/resources/sample-size-calculator/?conversion=1

http://optimizely-marketing-site-staging.s3-website-us-east-1.amazonaws.com/dfoxpowell/fix-sample-calculator-params/resources/sample-size-calculator/?conversion=21&effect=20

http://optimizely-marketing-site-staging.s3-website-us-east-1.amazonaws.com/dfoxpowell/fix-sample-calculator-params/resources/sample-size-calculator/?conversion=1&effect=20&power=80

http://optimizely-marketing-site-staging.s3-website-us-east-1.amazonaws.com/dfoxpowell/fix-sample-calculator-params/resources/sample-size-calculator/?conversion=1&effect=20&power=80&significance=83

http://optimizely-marketing-site-staging.s3-website-us-east-1.amazonaws.com/dfoxpowell/fix-sample-calculator-params/resources/sample-size-calculator/?conversion=1&effect=20&power=80&significance=83&tails=2